PROJECT SUMMARY Tetherin, also known as BST-2 or CD317, is an interferon-inducible transmembrane protein that inhibits the detachment of enveloped viruses from infected cells. Under conditions of interferon-induction, tetherin is upregulated on virus-infected cells and captures nascent virions as they attempt to bud from the cell surface. Whereas most simian immunodeficiency viruses (SIVs) use Nef to oppose the tetherin proteins of their nonhuman primate hosts, HIV-1 Vpu and HIV-2 Env have acquired the ability to counteract human tetherin because of the absence of a five amino acid sequence in human tetherin that confers susceptibility to Nef. We previously demonstrated that tetherin antagonism by Vpu protects HIV-infected cells from antibody-dependent cellular cytotoxicity (ADCC). We now show that the anti-tetherin activity of Vpu also protects HIV-infected cells from antibody-dependent cellular phagocytosis (ADCP). These findings imply that by trapping virions on the cell surface, tetherin increases the sensitivity of HIV-infected cells to Fc-mediated antibody responses, and that the antiviral activity of tetherin may be much greater in vivo than previously appreciated. The current proposal builds on these studies to address the overarching hypothesis that tetherin serves as link between innate and adaptive immunity to enhance the susceptibility of virus-infected cells to antibodies. In Aim 1, we will determine the immunological mechanisms by which tetherin enhances antibody-mediated phagocytosis of HIV-infected cells. These studies will focus on the factors that influence the extent to which tetherin can promote ADCP, which will provide a better understanding of how to use these interactions to improve antibody-based treatments for HIV-1 infection. In Aim 2, we will take advantage of the power of SIV infection of the rhesus macaque as an animal model to assess the contribution of viral countermeasures to tetherin and SERINC5 to lentiviral replication and pathogenesis. These studies will reveal the impact of tetherin and SERINC5 on lentiviral infection and the therapeutic benefit that may be derived from antiretroviral drugs designed to increase the sensitivity of HIV-1 to these restriction factors. In Aim 3, we will test the hypothesis that tetherin enhances antibody-mediated control of virus replication in SIV-infected macaques. These studies are fundamental to our basic understanding of the synergy between tetherin and antibodies and the potential to exploit these interactions for the treatment and prevention of HIV-1 infection.